The concepts of resonance and high frequency operation are being increasingly applied to the design of DC/DC power converter circuits. High frequency operation permits significant physical size and weight reduction of inductive and capacitive energy storage components. In one particular category of power supplies, referred to in one case as resonant converters and in another case as quasi-resonant converters, inductor-capacitor pairs operating at or near their resonance frequency, have been utilized to reduce switching losses in power switching transistor(s) by excluding the simultaneous presence of voltage and current during switching transitions. In some converter s L-C pairs have been used to regulate output voltage and in that capacity have the advantageous effect of reducing the bandwidth of energy transfer in the converter
One approach to resonant DC to DC power conversion, disclosed in U.S. Pat. No. 4,607,323, combines a single ended inverter having zero voltage switching transitions with a full wave rectifier. A matching network coupling the inverter to the rectifier controls the output impedance of the inverter to enable it to achieve the zero voltage switching operation so that switching loss in the switching transistor is significantly reduced by substantially eliminating the simultaneous occurrence of voltage and current in the switching transistor during on-off and off-on switching transitions. However, the switching transistor is subjected to very high peak voltages during its nonconduction interval in each cycle of operation that greatly exceed the input voltage applied to it.
Another approach to achieving reduced switching loss in the switching transistor of a single ended power converter is disclosed in U.S. Pat. No. 4,415,959 which achieves high efficiency by operating with zero current switching in the switching transistor and by substantially eliminating the simultaneous occurrence of voltage and current waveforms therein. This arrangement, however, limits the maximum frequency at which the converter may be operated because of losses which increase as a function of frequency and which are incurred by the discharge of the power transistor's shunt capacitance. An alternative to this arrangement is the bridge type inverter which has a lower voltage stress across the power switches. However a conventional bridge type such as a half-bridge inverter cannot be efficiently used as an inverter for application in a high-frequency DC/DC converter at high frequency switched voltages because energy is stored in the shunt capacitance of the switching transistors (normally FETs) and is then dissipated in the switching transistors.
In yet another approach, disclosed in the Apr. 1987 HFPC proceedings (B. Carsten--"A Hybrid Series-- Parallel Resonant Converter For High Frequencies And Power Levels", pg. 41-47), low voltage stress of the FET switches and zero voltage switching are achieved by using a half-bridge arrangement and controlling the switch off-time overlap.
Therefore the half bridge inverter as disclosed by Carsten is constructed so as to be operative so that both switching transistors are caused to be cyclically simultaneously nonconducting for a controlled period of time. This operational condition assures that energy stored in one transistor switch capacitance is transferred to the other transistor switch capacitance without any appreciable energy dissipation in the switch. The load presented to the inverter is inductive at the operating frequency. Hence, by controlling an interval of simultaneous nonconduction of the two switching transistors, a controlled current flowing during the off time overlap is operative to discharge the energy stored in one switching transistor capacitance into the other switching transistor capacitance.
A critical aspect of the high frequency converter is the rectification process. The rectifying action of the rectifying diodes produces ringing signals having high harmonics. The existence of parasitic elements in the rectifier adds to this ringing signal generation. These high frequency signals circulate throughout the rectifier and may cause significant power loss. If the input impedance of the rectifier is tuned in a straight forward manner to assure a resistive input impedance, the input resistance tracks the load resistance. If frequency shift control is used as a regulation technique, that input resistance tracking characteristic requires a substantial bandwidth of frequency control for a given regulation range.
In the converter designs mentioned above, as well as in many similar published designs, while inverter switching loss has been significantly reduced and switch voltage stress has been minimized, none of the designs has addressed the problem of taking the high-voltage, high-frequency output of the inverter and optimally transforming and rectifying it to obtain a low voltage DC output. Optimization in this sense means, in part, obtaining the desired range of input and output regulation with a minimum of frequency shift in the inverter, as well as a minimum of dissipation loss in the inverter, and in the transformation and rectification components. These desired results are achieved by the proper selection of the rectification and transformation means and by a matching of the rectification and transformation means to the inverter.